The present invention relates to a semiconductor device which includes a semiconductor region having a potential locally susceptible to change by minority carriers and a MOS transistor arranged around the semiconductor region.
A MOS transistor which operates in a saturation region is known to emit minority carriers into a semiconductor substrate from a depletion layer near the drain [Proceedings of the 11th Conference on Solid State Devices; Japanese Journal of Applied Physics. Vol. 19 (1980), Suppl. 19-1, pp 93 to 97].
The mode of operation of such a MOS transistor will now be described with reference to an n-channel MOS transistor shown in FIG. 1. When the MOS transistor shown in FIG. 1 operates in a saturation region, a channel current I1 flows from a source region 2 to a drain region 4 through a channel path 6 which is pinched off at the intermediate portion thereof and a depletion layer 8 formed in the surface area of a p-type substrate 10. The channel current I1 then flows to ground through the drain region 4 and a battery E1. In this case, a high electric field is established at that part of the depletion layer 8 which lies between the pinched off portion of the channel path 6 and the drain region 4. When the channel current I1 flows through the part having this high electric field, impact ionization is caused within the depletion layer 8 to generate hole-electron pairs. The holes generated in this manner flow to ground through the substrate 10 as a substrate current I2. Some of the electrons emitted by the impact ionization within the depletion layer 8 are attracted to the drain region 4 under the influence of the high electric field, while the remaining electrons diffuse into the substrate 10. The electrons injected into the substrate 10 in this manner continue to spread until they recombine with holes in the p-type substrate 10. Such electrons as to move in the substrate 10 tend to be more readily emitted from an output MOS transistor with a wide channel path or from a MOS transistor to which a drain voltage, raised by a bootstrap circuit or the like, is applied.
In a semiconductor element such as a semiconductor memory, especially a dynamic memory or a CCD image pick-up element an MOS transistor emitting minority carriers or electrons into that semiconductor substrate during operation in the saturation region is sometimes arranged near a semiconductor region within the semiconductor substrate which is set in a potential state which is not equilibrium. As used herein, such a semiconductor region is defined as a semiconductor region having a potential which is locally susceptible to change by minority carriers. Minority carriers or electrons which diffuse into the semiconductor substrate from the MOS transistor may flow into a potential well formed within the semiconductor region. This causes a change in the memory state or analog charge amount which can result in an erroneous operation of the semiconductor element.
When the minority carriers from the MOS transistor diffuse into the potential well of the semiconductor region set in the inequilibrium potential state, the potential well which has been empty is filled with the electrons and the stored data is thus inadvertently changed. This is described, for example, in "IEEE Trans. Electron Devices. ED-26 (1979) 1684". This erroneous operation is most likely to be caused in a memory cell which is close to the MOS transistor. In a case, for example, where the size of the memory cell is 10.times.10 .mu.m.sup.2, the thickness of the gate oxide film of the memory cell is 400 .ANG., the refresh cycle for the memory cell is 5 m sec. and the operation voltage is 5 V, the operation of the memory cell may be affected if a minority carrier current of more than 1.times.10.sup.-10 A flows into the memory cell area.
In the prior art, in order to solve this problem, a MOS transistor which causes a carrier current flow which may adversely affect the operation of the memory cell or which emits minority carriers of more than, for example, 1.times.10.sup.-10 A, is arranged at a distance from the memory cell. The distance is, for example, selected to be 50 .mu.m which is the diffusion length of electrons or longer. When these measures are taken, the minority carriers such as the electrons emitted from the MOS transistor recombine with the holes within the substrate before they reach the memory cell, so that the adverse effects of the carrier current of the MOS transistor may be eliminated. However, this requirement of arranging a MOS transistor at a distance from the memory cell results in a low packing density.